Kinetic Isotope Effect in Photochlorination of H2, CH4, CHCl3, and C2H6

Abstract

The relative rates of hydrogen and deuterium abstraction by chlorine atoms from H2–D2, CH4–CD4, and C2H6–C2D6 in direct competition have been studied by a mass spectrometer technique permitting direct determinations of rates. The pair CHCl3–CDCl3 was investigated by a substitution—addition competition with C2Cl4. Data were obtained between 300° and 475°K, and values of kH/kD ranged between 3.64 and 10.9 for CH4/CD4, 2.37 to 2.69 for C2H6/C2D6, 1.58 to 2.26 for CHCl3/CDCl3, and 4.7 to 8.9 for H2/D2. For the family of reactions involving C–H–Cl, an extensive series of model calculations were made by the ``bond-energy—bond-order'' method. The expected variation of activation energy with heat of reaction was computed and compared with experiment. The expected variation of kinetic isotope effect with both temperature and heat of reaction was computed for a range of conditions much wider than these experiments. Simple activated complex theory predicts several strong trends of kinetic isotope effect with heat of reaction. The data, in a general semiquantitative way, follow these predicted trends, but in terms of fine details the data do not fit the theoretical lines. In view of the known simplifications in activated complex theory, this agreement with broad trends is all that anyone should expect.

Competitive photochlorination and kinetic isotope effects for hydrogen/deuterium abstraction from the methyl group in C2H6, C2D6, CH3CHCl2, CD3CHCl2, CH3CCl3, and CD3CCl3

“…Our data on the C2H6/C2D6 system are in close agreement with those of Chiltz et al (10). The isotope effect is rather weak compared to the so-called "theoretical maximum", which is based on a one-dimensional three-particle model but which ignores any tunneling correction.…”

“…Chiltz et al (10) reported the rate constant ratios, kl/k2, at several temperatures in the range 302-433 K for the direct competition in C2H6/C2D6 mixtures using a mass spectrometric technique. Although these data show a high degree of scatter when plotted according to the Arrhenius rate law, an approximate temperature dependence is given by In (kl/kl) = 0.53 + 137/T, in satisfactory agreement with our value of In (k,/kz) = 0.545 + 151/T based on the data in Table I.…”

Primary kinetic isotope effect in the gas phase photochlorination of ethyl chloride-1-d1

“…For instance, both the magnitude and the trend in activation energy with the heat of reaction for chlorine atom attack in chloromethanes and chloroethanes is predicted, contrary to experiments (6). Furthermore, the chlorination of C2H6/C2D6 is one of the few examples of the failure of the BEBO method for the calculation of kie (3,7). In the case of ethyl chloride the situation might have been expected to be even worse based on the arguments of Johnston and Goldfinger (6) that the preferential attraction of the chlorine atom by the bound chlorine may result in collision complexes of the form C-Cl---Cl superseding those of the form C-H---Cl to the extent that in the case of a molecule such as CHCl, a hydrogen atom transfer complex may never be formed.…”

“…In the case of several classes of reactions to which abstractions by chlorine atoms belong, the BEBO method is known to fail (3,5). Generally, activation energies can be calculated to within 1 or 2 kcal/mol compared with the experimental data, whereas abstractions of hydrogen by chlorine atoms are predicted to have activation energies much higher than those observed, frequently by [6][7] For personal use only.…”

“…In the latter case, the potential energy field around the atom being transferred and the atomic masses involved are the crucial factors. An extensive study of kinetic isotope effects in the photochlorination of hydrogen, methane, methyl chloride, and ethane was reported by Chiltz et al (7). The BEBO method was applied to calculate activation energies, kinetic isotope rate constant ratios, and their temperature coefficients, but met only with partial success.…”